'boxer' originates from English, specifically from the verb 'box' plus the agentive suffix '-er', where 'box' meant 'to strike with the fist'.

'box' appears in Middle English as 'boxen' meaning 'to strike'; the agent noun 'boxer' was formed by adding '-er' to denote 'one who boxes'. Over time 'boxer' came to denote a competitor in the sport of boxing and later was applied to the style of shorts worn by such athletes.

Initially it meant 'one who strikes (with the fist)'; it evolved into the specific sense 'a person who competes in boxing' and by extension also came to denote 'loose-fitting shorts associated with boxers'.